Your brain gets smaller as you sleep (and you forget things)

Every day, as we learn new things and make new memories, our brains physically swell.

And at night, as we sleep, those memories are pruned and our brains shrink a little.

“Maybe you had a girlfriend or boyfriend 20 years ago that you rarely think about. Every night, the memory of that distant partner gets a little bit weaker, a little bit fainter, until it fades away to nothing,” says Dr John Lesku, one of the world’s leading experts on sleep evolution.

But evolutionary biologists don’t see it that way. To them, sleep is ostensibly pretty dumb.

When an animal sleeps, it is at its most vulnerable – defenceless and unaware. Plus sleep is essentially wasted time when a creature could be eating or mating. “It’s a really bad idea,” says Dr Lesku, who leads a sleep research group at La Trobe University.

Sleep is extraordinarily inefficient, and natural selection has a particular mean streak for inefficiency.

But every animal sleeps. In fact, many of the simplest animals, with neural systems we would not recognise as brains, sleep more than humans.

Roundworms have about 300 neurons and no brain to speak of, and they sleep. Jellyfish have a ‘nerve net’, a ring-shaped set of neurons, and they sleep.

“We have a mechanical device that just jolts them – we keep them awake all night,” he says.

Flies have about 100,000 neurons in their brain. Humans have something like 100 billion.

“So you’d think humans need more sleep, but a fly needs just as much – if not more,” he says.

If simple, ancient animals like worms, flies and jellyfish need their shut-eye, it suggests sleep evolved very early on in life. And brains are not required (at least, not for all sleep's functions).

Dr John Lesku studies sleep in animals at La Trobe UniversityCredit:Jason South

“These are brainless animals, sleeping,” says Dr Lesku. “It means the biological target of sleep function is not the brain, as we once thought, but is probably something like a neuron or synapse.”

The military has been trying to find a way to let soldiers go sleepless for days. “But they have never been able to,” says Professor van Swinderen.

Sleep performs something so vital to life neither natural selection nor the military-industrial complex have been able to get rid of it.

The theory scientists have come up with in the last decade to explain this is remarkable.

Humans, like flies, sleep in stages. There is rapid-eye-movement sleep. There is dream sleep. And there is deep sleep.

In 2003, sitting around the lunch table in a break room at the University of Wisconsin, Giulio Tononi and Chiara Cirelli were kicking around a new discovery: if you subject networks of neurons in lab dishes to electrical pulses at 1 hertz, the connections between them seem to weaken.

Wasn’t it remarkable, someone pointed out, that 1 Hz was the exact same speed brain waves slowed to during deep sleep? The two scientists looked at each other. Wasn’t it, indeed?

Their theory, ‘sleep homeostasis’, caused a huge stir when it was published in 2003, in part because it was so weird. A decade later it is now a mainstream theory. Here's how it works.

Every day as the brain is exposed to new stimuli, the connections between the most-used neurons grow physically thicker. As you practice guitar, the connections involved get stronger - and you get better.

We have about one trillion neuron-connections (known as synapses) in our brain. If each were to grow by just 1 per cent a day, “that would be unsustainable – the brain would explode after a couple of days,” says Professor van Swinderen.

“Sleep is the solution to that.”

Every night during deep sleep, the theory posits, every connection in the brain grows weaker. The brain grows physically smaller. Every night, the memory of your ex-partner grows a little weaker, until one day it is gone.

Every day the most-used connections grow strong, and every night every connection grows weaker. Over time only the most-used connections remain strong. This is why you can learn the guitar – because your brain is freeing up space by forgetting your ex-girlfriends.

Well, maybe.

Professor van Swinderen studies flies, and Dr Lesku studies, among other beasts, the elegant crested tinamou, a fast-running bird with a head plumage that curls like the eyebrow of an extremely-unimpressed high school teacher.

Ethical guidelines prevent researchers from peeling off the skull of sleeping humans to get at the neurons, meaning nearly all such research is done in animals.

So synaptic homeostasis, in humans at least, remains a theory, and a controversial one at that.

And even if it proves true, it only explains one part of sleep’s function. Animals have rapid-eye-movement sleep too. Reptiles even have something that looks a lot like dream-sleep.